The evaluation of a response of a substance to an excitation acting upon the substance enables the determination of characteristic quantities of the substance corresponding to substance properties and substance parameters in the analysis interval covered by the observation. The substance subjected to the excitation may be a pure substance as well as a system or mixture of substances or materials. A widely known example of such a method is dynamic thermal analysis, in which the excitation is effected via a predetermined temporal temperature program to which the substance is subjected and in which the heat flow produced by the sample is captured as a response. Often, this process is implemented as a differential method in which the substance and a known reference substance are excited in accordance with the temperature program and the difference between the heat flows produced by the substance and the reference substance is used as the response. Another well-known example is the thermo-mechanical analysis, in which the response is observed in terms of a change in length of the body of a sample substance, as a function of a predetermined temperature program.
For a known differential thermal analytical method (Euro. Pat. Pub. No. EP 0 559 362 A1), the temperature program implementing the excitation consists of a ramp ascending in a linear manner, which is superimposed by a periodic temperature modulation of preset frequency and preset amplitude. The evaluation of the modulated heat flow difference obtained as the response is based on the split of the response signal representing this heat flow difference into two signal components. One of the signal components is obtained by averaging over one or several modulation periods, respectively, i.e., it constitutes a common component contained in the response signal. The other signal component is the alternating component contained in the response signal, with said alternating component oscillating with the preset modulation frequency and being established by determining the difference between the measured response signal and its common component. This type of excitation and evaluation of the response signal is based on the use of a single preset modulation frequency, with only such events being selectively excited which belong to the same frequency or its harmonic waves.
This restriction to a single excitation frequency is avoided by a different generally known thermal analytical method (Euro. Pat. Pub. No. EP 1 091 208 A1), which provides for a stochastic excitation and subjects the response signal to a correlative analysis during its evaluation. However, the required measuring time is increased in the correlative analysis when a high degree of accuracy is needed.